1. Field of the Invention
The present invention relates to a method of continuous heat treatment designed to improve the quality of the surface of the sheet, in particular cold rolled steel sheet.
2. Description of the Prior Art
In industrial practice, cold rolled steel sheet is obtained (after preparation of the steel and hot rolling of the strand or slab) by pickling of the hot-rolled strip followed by cold rolling to the required thickness and finally by annealing in order to restore the mechanical properties of the steel and by a skin-pass to provide the steel with the required final surface finish and to remove the yield plateau of the tensile test curve.
All the operations following hot rolling have an effect on the final surface condition of the sheet. Thus, inadequate rinsing after pickling may provide the possibility of subsequent contamination. In the same way, the selection of the rolling oil is extremely important to the extent that this oil may not be removed from the surface of the sheet if the annealing process is not suitably adapted to this.
Several authors have given sufficient proof of the fact that the surface cleanness of steel sheet (more particularly the amount of carbon deposited) is an important parameter in explaining the suitability of this sheet for phosphate coating and its resistance to corrosion by salt spray after painting. Such surface cleanness may be tested in several ways, for example by the adhesive tape test in which transparent adhesive tape is applied to the surface of the sheet and then removed with possible deposits taken from the sheet. It is possible to measure the absorption of the light passing through the tape and therefore to quantify the surface deposits on the sheet. A method of this type provides a measurement of the amounts of deposits of all types on the surface, for example, dust, carbon traces, filings, etc.
A further method of testing the surface quality, which is also extremely widespread, consists in quantifying the total amount of carbon present on the surface of the steel. This involves washing the surface of the sheet with hydrochloric acid by means of pads of inorganic material which is then "burnt" with oxygen and the amount of CO.sub.2 released is measured. It is therefore possible to measure the total amount of carbon present in various forms on the surface of the steel in mg/m.sup.2. It is also possible, for the purpose of standardizing tests, to utilize a power wash (with jets) before the hydrochloric acid washing in order to remove possible protective oils and to bring the sheet into the condition which it possesses after shaping and before phosphate coating and final painting. This is the case in the well-known as the "Ford test."
Further means of analysing the surface of steel are provided by the ion microanalyser, the Auger spectrometer, etc. These enable detection of all the chemical elements on the surface and the development of their concentration as a function of depth.
These techniques enable the detection of possible contamination by elements other than iron, these elements possibly being a result of the baths used (washing, pickling, rinsing, degreasing) or possibly being due to the steel itself.
As stated above, it is known at present that surface carbon on steel impairs resistance to corrosion by salt spray applied to the painted sheet. This carbon is deposited chiefly by the rolling oil. In current practice, the rolling oil is not removed from the surface of the sheet after rolling, but is evaporated during batch annealing. However, when the amount of surface carbon is measured after annealing of this type, it is possible to observe considerable contamination which leads to unfavorable phosphate coating and painting (exposure to salt spray) results.
Considerable progress has been made by subjecting the product to continuous annealing preceded by degreasing, for example by electrolysis in a solution of sodium orthosilicates.
In the case of simple continuous annealing, heating is in effect carried out under an N.sub.2 /H.sub.2 atmosphere and the oil does not have the time to evaporate, as the heating is very rapid. On the other hand, in several known methods, continuous annealing is preceded by a degreasing operation which is effected, in the majority of cases, in an alkaline medium. As the rolling oil has been eliminated before the sheet is placed in the furnace, the surface cleanness is considerably greater, in particular in respect of the total amount of surface carbon, which is decreased for example to 1 mg/m.sup.2 to 8 mg/m.sup.2 in the case of very clean sheet produced in a static furnace. However, as stated above, while such a decrease in the amount of surface carbon should, according to various authors, lead to an improvement in painting results, it has been observed that this improvement is not particularly great.